From prior art, there have been known electrically driven vehicles having an electric motor, which is powered by electric energy charged in a battery in order to serve as a drive power source. Such electrically driven vehicles include so-called electric vehicles (EVs) having an electric motor as the only drive power source, as well as so-called hybrid electric vehicles (HEVs) having an electric motor and an internal combustion engine as the drive power sources.
Many electrically driven vehicles have a regenerative braking system, in which the electric motor is used as an electric generator, which is driven to provide braking force (regenerative braking force) so as to decelerate the vehicle, while the kinetic energy of the vehicle is converted into electric energy to achieve energy recovery. For higher energy efficiency, it is desirable to select a higher regenerative braking force, which results in more regenerative electric energy recovered. Too high a regenerative braking force, however, may result in too rapid deceleration of the vehicle, which may negatively affect on the drivability of the vehicle.
In order to address the problem, there is a technique well known in the art, in which an operation unit is provided for the driver to select a magnitude of the braking force to be generated during generative braking.
For example, Patent Document No. 1 listed below (Japanese Patent Publication No. 3,714,164) discloses a vehicle including an electric motor, a transmission with selectable gear positions of different gear ratios, and a drive shaft. These elements are connected with one another such that the torque of the electric motor is used for braking purpose. The vehicle further includes an operation unit for the driver of the vehicle to demand increase/decrease of the deceleration rate to be achieved during braking by the electric motor; means for calculating a desired deceleration rate by effecting increase/decrease adjustment to a predetermined initial default deceleration rate in accordance with the operation to the operation unit; means for selecting a desired gear position for achieving the desired deceleration rate by the torque of the electric motor; means for determining a desired operation condition of the electric motor for providing the drive shaft with braking force so as to achieve the desired deceleration rate; and a controller means for controlling the transmission to shift it into the desired gear ratio as well as for controlling the electric motor to operate it under the desired operation condition.
In addition, Patent Document No. 1 describes a failure detector for detecting any failure in the operation unit and resetting the desired deceleration rate when a failure in the operation unit is detected.